1. Field
A weighing module, and a device that is used to weigh substantially uniform weighing objects with a given number of weighing modules, are disclosed. Each module has a load receiver that is connected to a weighing cell through a force-transmitting rod. A parallel-guiding mechanism is associated with each force-transmitting rod to constrain the latter in a parallel-guided movement in the direction of the load.
Applications for a device designed to weigh substantially uniform weighing objects are found in automated production- and testing systems where balances of a modular configuration—so-called weighing modules—are particularly well suited to be integrally incorporated into these systems. In essence, the balances used for this purpose are of the type where the indicator unit is arranged separately from the balance, for example in a system with a central indicator unit for a plurality of weighing modules. Integrated weighing modules of this kind are used in systems for the production and testing of small and relatively expensive parts, for example in filling- and packaging machines for tablets, capsules, ampoules, etc. in the pharmaceutical industry, or in the checking of ball bearings. The weighing of substantially uniform objects and also the so-called batch-weighing are processes in which a plurality of load quantities have to be weighed individually, be it for the purpose of checking, dosage-dispensing, or filling, etc. within a limited space.
Since a conveyor device such as a robotic arm with multiple grippers can be used to put the weighing objects onto the individual load receivers of the weighing modules and to remove them from there after they have been weighed, the positions of the individual load receivers in relation to each other and in relation to the conveyor device have to be accurately and durably set.
2. Background Information
Devices which are used for weighing substantially uniform weighing objects are known. Predominantly, these devices are arrangements of weighing modules in rows or two-dimensional arrays. Other arrangements are based on the concept of placing the weighing modules in a two-dimensional satellite-like arrangement around a row arrangement of load carriers, wherein the latter have to be matched to the distances between the delivery elements of an existing conveyor device, because the weighing module is often too large to allow an arrangement at the required close intervals.
A two-dimensional arrangement of weighing cells is disclosed in JP 01212327 A, which describes a method of using a plate of a spring material to produce a large number of weighing cells carrying strain gauges as sensor elements. However, strain-gauge-based weighing cells—in contrast to weighing cells based on the principle of electromagnetic force compensation—may not suitable for applications involving the determination of a mass in the range of micrograms to grams.
A device for the gravimetric testing of multi-channel pipettes is disclosed in DE 299 17 940 U1. The device has a plurality of weighing cells arranged in a plane either in a satellite-like layout or side-by-side. To provide simultaneous weighing of the test volumes of a multi-channel pipette, the load receivers of the weighing cells are arranged close together. A satellite-like layout makes it possible to use relatively large weighing cells for the testing of multi-channel pipettes.
A weighing cell based on the principle of a single string oscillator is disclosed in CH 654 412 A5. Using two spring elements of a meandering shape, the load receiver is guided in parallel motion relative to the console. A factor of significant influence on the measurement accuracy of these weighing cells is that the force being measured is always applied in the same direction to the string. With the meander-like arrangement of the guide arms, the changes in length of the guide arms compensate each other so that the position of the load receiver relative to the console does not change with temperature variations.
In a weighing cell that functions according to the principle of electromagnetic force compensation, the force that is caused by a load on the weighing pan can be compensated by a force-compensating member having a permanent magnet and a coil, wherein the current is measured which flows through the coil to generate the compensating force. The measured value is in proportion to the load placed on the weighing pan. However, the measured value is also dependent on the position of the coil in the magnetic field of the permanent magnet and therefore, when determining the measurement value, the coil has to have the same position in relation to the magnet. The position of the coil after applying the load is determined by way of a position sensor, and the current through the coil is increased until the load-related displacement of the coil in relation to the permanent magnet is compensated. At this point the coil current is measured, which represents a measure for the weight of the applied load. A weighing cell of this type is disclosed in CH 638 894 A5, wherein the weighing cell has a force-transmitting device which is arranged between the load receiver and the force-compensating member and which transmits the force generated by the load on the load receiver to the force-compensating member, reducing or magnifying the force depending on the load range.
A weighing cell that works according to the same principle is disclosed in CH 593 481 A5. In this patent, the load receiver is coupled directly to the force-compensating member by way of a force-transmitting rod. The movable part of the position sensor is attached to the force-transmitting rod, while the stationary part of the position sensor is rigidly connected to the housing-based part of the weighing cell, or generally to the stationary part of the force-compensating member. This arrangement which is referred to as direct-measuring principle can be used in the range of small loads. As the position sensor has only a limited resolution, the precision of the measurement depends essentially on the resolution of the position sensor.
The load receiver and the coil of the force-compensating device are precisely guided in relation to the stationary part of the weighing cell. This can be accomplished by a parallel-guiding mechanism whose movable parallel leg is connected to the force-transmitting rod and whose stationary parallel leg is rigidly connected to the housing-based part of the weighing cell. The movable parallel leg and the stationary parallel leg are connected to each other through two parallel-guiding members that are rigid against bending and have thin flexure joints. However, one could also use spring-like elastic parallel-guiding members, in which case the thin flexure joints are omitted. When a load is placed on the load receiver, the force-transmitting rod moves in the direction of the load, whereby the parallel-guiding members are deflected and the thin flexure joints or spring-like elastic parallel-guiding members are caused to bend. Analogous to a leaf spring element, these thin flexure joints or spring-like elastic parallel-guiding members generate a moment of a magnitude that is in proportion to the angle of deflection of the parallel-guiding members and acts in the opposite direction of the bend, or a force that acts in the opposite direction of the load. The more massive the thin flexure joints are designed, the larger is the load differential that is needed to produce the minimally detectable displacement of the position sensor. Thus, the dimensions of the flexure joints or the elastically flexible parallel-guiding members can also significantly influence the resolution of the weighing cell.
The parallel-guiding mechanisms disclosed in the state-of-the-art references have a disadvantage that the maximally tolerable stress in the material being used imposes a limit on reducing the thickness of the thin flexure joints and that the parallel-guiding mechanism becomes very sensitive to overloads if the flexure joints or the elastic parallel-guiding members are made thinner. This can be alleviated by making the parallel-guiding members longer. This reduces the amount of angular movement at the flexure joints for the minimal displacement that can be detected by the position sensor. However, this can lead to unfavorable dimensions for weighing modules that are to be used in a device for weighing uniform weighing objects, so that the devices become expensive, voluminous and complicated.